Coupled General Circulation Model Research Articles

The Euro-Atlantic Circulation Response to the Madden-Julian Oscillation Cycle of Tropical Heating: Coupled GCM Intervention Experiments

Intervention experiments using the Coupled Forecast System model, version 2 (CFSv2), have been performed in which various Madden-Julian Oscillation (MJO) evolutions were added to the model’s internally generated heating: Slow Repeated Cycles, Slow Single Cycle, Fast Repeated Cycles, and Fast Single Cycle. In each experiment, one of these specified MJO evolutions of tropical diabatic heating was added in multiple ensemble reforecasts of boreal winter (1 November to 31 March for 31 winters: 1980–2010). Since in each experiment, multiple re-forecasts were made with the identical heating evolution added, predictable component analysis is used to identify modes with the highest signal-to-noise ratio. Traditional MJO-phase analysis of total model heating (dominated by internally generated heating) shows that the MJO-related heating structure compares well with heating estimated from observed fast and slow episodes; however, the model heating is larger by a factor of two. The evolution of Euro-Atlantic circulation regimes indicates a clear response due to the added heating, with a robust increase in the frequency of occurrence of the negative phase of the North Atlantic Oscillation (NAO−) after the heating crosses into the Pacific and a somewhat less robust increase in the positive phase of the NAO (NAO+) following Indian Ocean heating. In the Fast Cycle experiments, the model response is somewhat muted compared with the Slow Cycle experiments. The Scandinavian Blocking regime becomes more frequent prior to the NAO− regime. The two leading modes in the predictable component analysis of 300 hPa height (Z300), synoptic scale feedback (DZ300), and planetary wave diabatic heating in all experiments form an oscillatory pair with high statistical significance. The oscillatory pair represents the cyclic response to the particular MJO signal (Fast or Slow, Single, or Repeated Cycles) in each case. The period is about 64 days for the Slow Cycle and 36 days for the Fast Cycle, consistent with the imposed periods. The time series of one of the leading modes of Z300 is highly anti-correlated with the frequency of occurrence of the NAO– in the Repeated Cycle experiments. A clear cycle involving the Z300 and DZ300 leading modes is identified.

Jet–Precipitation Relation and Future Change of the Mei-Yu–Baiu Rainband and Subtropical Jet in CMIP5 Coupled GCM Simulations

Abstract Through extensive modeling efforts, it has been established that the ongoing global warming will increase the overall precipitation associated with the East Asian summer monsoon, but the future change of its spatial distribution has not reached a consensus. In this study, meridional shifts of the mei-yu–baiu rainband are studied in association with the subtropical jet by using outputs from atmosphere–ocean coupled climate models provided by CMIP5. The models reproduce observed associations between the jet and precipitation over wide time scales from synoptic to interannual. The same relation is found in intermodel differences in simulated climatology, so that the meridional locations of the jet and baiu precipitation are positively correlated. The multimodel-mean projection suggests that the both are shifted southward by the late twenty-first century. This shift is not inconsistent with the projected tropical expansion, not only because the change is local but also because the projected tropical expansion occurs mainly in the Southern Hemisphere. No significant future change in the continental mei-yu precipitation location is identified, which might be because the jet change is weak there. For comparison, the summertime Atlantic jet position, which shifts northward, is investigated briefly. This study suggests that the future change of the subtropical jet is an important aspect to investigate possible future changes of the baiu rainband, and it prompts further studies including the role of the ocean.

Frontogenesis of the Angola–Benguela Frontal Zone

Abstract. A diagnostic analysis of the climatological annual mean and seasonal cycle of the Angola–Benguela Frontal Zone (ABFZ) is performed by applying an ocean frontogenetic function (OFGF) to the ocean mixing layer (OML). The OFGF reveals that the meridional confluence and vertical tilting terms are the most dominant contributors to the frontogenesis of the ABFZ. The ABFZ shows a well-pronounced semiannual cycle with two maximum (minimum) peaks in April–May and November–December (February–March and July–August). The development of the two maxima of frontogenesis is due to two different physical processes: enhanced tilting from March to April and meridional confluence from September to October. The strong meridional confluence in September to October is closely related to the seasonal southward intrusion of tropical warm water to the ABFZ that seems to be associated with the development of the Angola Dome northwest of the ABFZ. The strong tilting effect from March to April is attributed to the meridional gradient of vertical velocities, whose effect is amplified in this period due to increasing stratification and shallow OML depth. The proposed OFGF can be viewed as a tool to diagnose the performance of coupled general circulation models (CGCMs) that generally fail at realistically simulating the position of the ABFZ, which leading to huge warm biases in the southeastern Atlantic.

Climate model projections for future seasonal rainfall cycle statistics in Northwest Costa Rica

Increasing water scarcity due to rising demand and changes in climate and land use are expected to exert significant stress on water resources in many parts of the world. In many areas, distinctive patterns of seasonal precipitation play an important role in regional ecosystems, economies, and food and energy supplies. Given the inconsistency among climate model precipitation projections, this study develops a statistical approach to better characterize seasonal patterns of precipitation and to assess the potential impact of climate change on these patterns. The method is applied to evaluate the bimodal seasonal pattern of precipitation in northwest Costa Rica as a case study. A Gaussian mixture model is used to describe the bimodal pattern and quantify changes in the seasonal precipitation cycle projected by 19 Coupled General Circulation Models. The model simulations for the current period (1979–2005) are compared with observed monthly precipitation data based on four goodness‐of‐fit metrics to select the best performing models. The monthly bias‐corrected and spatially disaggregated (BCSD) climate projections from the selected models are used to investigate the projected change in the bimodal seasonal pattern, seasonal mean precipitation and interannual variability at the late twenty‐first century. Under RCP8.5, the degree of consistency associated with these BCSD climate projections is found to be higher for the projections of the midsummer drought than that for the early portion of the wet season (early season). For the late portion of the wet season (late season) projections, the degree of consistency is found to be the lowest. The proposed method provides (a) an overall characterization of inconsistencies in climate model projections for the region, (b) more information about the possible changes in the seasonal cycle of precipitation, and (c) the possibility of performing detailed comparison tests among climate models over a set of simulations and projections.

Assessing Extratropical Influence on Observed El Niño–Southern Oscillation Events Using Regional Coupled Data Assimilation

The extratropical influence on the observed events of El Niño–Southern Oscillation (ENSO) variability from 1948 to 2015 is assessed by constraining the extratropical atmospheric variability in a coupled general circulation model (CGCM) using the regional coupled data assimilation (RCDA) method. The ensemble-mean ENSO response to extratropical atmospheric forcing, which is systematically and quantitatively studied through a series of RCDA experiments, indicates robust extratropical influence on some observed ENSO events. Furthermore, an event-by-event quantitative analysis shows significant differences of the extratropical influence among the observed ENSO events, both in its own strength and in its relation to tropical precursors such as the equatorial Pacific heat content anomaly. This study provides the first dynamic quantitative assessment of the extratropical influence on observed ENSO variability on an event-by-event basis.

ENSO influence over the Pacific North American sector: uncertainty due to atmospheric internal variability

The relative influence of El Nino Southern Oscillation (ENSO) forced response versus internally generated atmospheric variability or noise on the upper tropospheric Pacific North American circulation is investigated. The analysis is performed on the boreal winter (December–January–February) 200 hPa circulation and the associated precipitation based on observational records and modeling experiments. The model experiment includes an ocean eddy-resolving coupled general circulation model (CGCM) and an atmospheric noise reduced ocean eddy-resolving CGCM. The noise reduction technique is the interactive ensemble approach, adopted to reduce the effects of internal atmospheric dynamics noise at the air-sea interface. Tropical rainfall anomalies associated with ENSO forces a teleconnection pattern that is a combination of the so-called Pacific North American (PNA) pattern, themed here as state-dependent atmospheric noise, and a pattern distinct from the PNA, themed here as ENSO-signal. The ENSO signal has a meridional structure in the streamfunction associated with significant poleward Rossby wave flux emanating from the eastern Pacific. Conversely, the PNA teleconnection pattern is zonally oriented, with most of the wave flux in the zonal direction from the Pacific towards North America. The mid-latitude ENSO forced response is asymmetric between warm and cold events. This asymmetry is strongly dependent on the amplitude of atmospheric noise. It is shown that the ENSO forced response is masked by atmospheric noise, with the latter being 3–10 times larger in amplitude. We show that the PNA pattern was positive during the 2015–2016 boreal winter and prevented the large 2015–2016 El Nino event from alleviating the persistent drought in the western US.

A Role for the Equatorial Undercurrent in the Ocean Dynamical Thermostat

AbstractReconstructions of sea surface temperature (SST) based on instrumental observations suggest that the equatorial Pacific zonal SST gradient has increased over the twentieth century. While this increase is suggestive of the ocean dynamical thermostat mechanism of Clement et al., observations of a concurrent weakening of the zonal atmospheric (Walker) circulation are not. Here we show, using heat and momentum budget calculations on an ocean reanalysis dataset, that a seasonal weakening of the zonal atmospheric circulation is in fact consistent with cooling in the eastern equatorial Pacific (EEP) and thus an increase in the zonal SST gradient. This cooling is driven by a strengthening Equatorial Undercurrent (EUC) in response to decreased upper-ocean westward momentum associated with weakening equatorial zonal wind stress. This process can help to reconcile the seemingly contradictory twentieth-century trends in the tropical Pacific atmosphere and ocean. Moreover, it is shown that coupled general circulation models (CGCMs) do not correctly simulate this process; we identify a systematic bias in the relationship between changes in equatorial surface zonal wind stress in the EEP and EUC strength that may help to explain why observations and CGCMs have opposing trends in the zonal SST gradient over the twentieth century.

An examination of the Northern Hemisphere mid-latitude storm track interannual variability simulated by climate models\u2014sensitivity to model resolution and coupling

The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air–sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.

Coupling general ocean circulation models with the BFM marine ecosystem model

The coupling of numerical geophysical models of a sea with marine ecosystem models is considered. For the latter, BFM was chosen and coupled with three Russian models for three seas of different spatial scales: the White Sea (JASMINE), the Arctic Ocean (FEMAO), and the global ocean as a part of the Earth system (INMCM). We describe technical details of implementation and discuss important questions of the computing efficiency and sensitivity of the model to the sea-ice subsystem and boundary values. The ecology model can simultaneously embrace pelagic, benthic, and sympagic ecosystems.

Influences of the North Pacific Victoria Mode on the South China Sea Summer Monsoon

Using the reanalysis data and the numerical experiments of a coupled general circulation model (CGCM), we illustrated that perturbations in the second dominant mode (EOF2) of springtime North Pacific sea surface temperature (SST) variability, referred to as the Victoria mode (VM), are closely linked to variations in the intensity of the South China Sea summer monsoon (SCSSM). The underlying physical mechanism through which the VM affects the SCSSM is similar to the seasonal footprinting mechanism (SFM). Thermodynamic ocean–atmosphere coupling helps the springtime SST anomalies in the subtropics associated with the VM to persist into summer and to develop gradually toward the equator, leading to a weakened zonal SST gradient across the western North Pacific (WNP) to central equatorial Pacific, which in turn induces an anomalous cyclonic flow over the WNP and westerly anomalies in the western equatorial Pacific that tend to strengthen the WNP summer monsoon (WNPSM) as well as the SCSSM. The VM influence on both the WNPSM and SCSSM is intimately tied to its influence on ENSO through westerly anomalies in the western equatorial Pacific.

The Climatological Effect of Saharan Dust on Global Tropical Cyclones in a Fully Coupled GCM

AbstractClimate in the tropical North Atlantic and West Africa is known to be sensitive to both the atmospheric burden and optical properties of aerosolized mineral dust. We investigate the global climatic response to an idealized perturbation in atmospheric burden of Saharan‐born mineral dust, comparable to the observed changes between the 1960s and 1980s, using simulations with the high‐resolution, fully coupled Geophysical Fluid Dynamics Laboratory Climate Model 2.5, Forecast‐oriented Low Ocean Resolution version, across a range of realistic optical properties, with a specific focus on tropical cyclones. The direct radiative responses at the top of the atmosphere and at the surface along with regional hydrologic and thermodynamic responses are in agreement with previous studies, depending largely on the amount of aerosol absorption versus scattering. In all simulations, dust causes a decrease in tropical cyclone activity across the North Atlantic Ocean, as determined by a tropical cyclone tracking scheme, with the largest response occurring in the most absorbing and scattering optical regimes. These changes are partially corroborated by common local genesis potential indices. However, no clear‐cut explanation can be developed upon inspection of their constituent variables. There are also nonnegligible anomalies in the North Pacific and Indian Oceans in these simulations. A relationship between accumulated cyclone energy and top of the atmosphere radiative flux anomalies is used to explain the North Atlantic anomalies, while analogy to known climate variations can help us understand the far‐field response to the dust forcing.

SST biases over the Northwest Pacific and possible causes in CMIP5 models

In this paper, the features and possible causes of sea surface temperature (SST) biases over the Northwest Pacific are investigated based on a mixed-layer heat budget analysis in 21 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Inter-comparison Project (CMIP5). Most CMIP5 models show cold SST biases throughout the year over the Northwest Pacific. The largest biases appear during summer, and the smallest biases occur during winter. These cold SST biases are seen at the basin scale and are mainly located in the inner region of the low and mid-latitudes. According to the mixed-layer heat budget analysis, overestimation of upward net sea surface heat fluxes associated with atmospheric processes are primarily responsible for the cold SST biases. Among the different components of surface heat fluxes, overestimated upward latent heat fluxes induced by the excessively strong surface winds contribute the most to the cold SST biases during the spring, autumn, and winter seasons. Conversely, during the summer, overestimated upward latent heat fluxes and underestimated downward solar radiations at the sea surface are equally important. Further analysis suggests that the overly strong surface winds over the Northwest Pacific during winter and spring are associated with excessive precipitation over the Maritime Continent region, whereas those occurring during summer and autumn are associated with the excessive northward extension of the intertropical convergence zone (ITCZ). The excessive precipitation over the Maritime Continent region and the biases in the simulated ITCZ induce anomalous northeasterlies, which are in favor of enhancing low-level winds over the North Pacific. The enhanced surface wind increases the sea surface evaporation, which contributes to the excessive upward latent heat fluxes. Thus, the SST over the Northwest Pacific cools.

Estimating Convection Parameters in the GFDL CM2.1 Model Using Ensemble Data Assimilation

AbstractParametric uncertainty in convection parameterization is one major source of model errors that cause model climate drift. Convection parameter tuning has been widely studied in atmospheric models to help mitigate the problem. However, in a fully coupled general circulation model (CGCM), convection parameters which impact the ocean as well as the climate simulation may have different optimal values. This study explores the possibility of estimating convection parameters with an ensemble coupled data assimilation method in a CGCM. Impacts of the convection parameter estimation on climate analysis and forecast are analyzed. In a twin experiment framework, five convection parameters in the GFDL coupled model CM2.1 are estimated individually and simultaneously under both perfect and imperfect model regimes. Results show that the ensemble data assimilation method can help reduce the bias in convection parameters. With estimated convection parameters, the analyses and forecasts for both the atmosphere and the ocean are generally improved. It is also found that information in low latitudes is relatively more important for estimating convection parameters. This study further suggests that when important parameters in appropriate physical parameterizations are identified, incorporating their estimation into traditional ensemble data assimilation procedure could improve the final analysis and climate prediction.

Month-to-month variability of Indian summer monsoon rainfall in 2016: role of the Indo-Pacific climatic conditions

Indian summer monsoon (ISM) rainfall during 2016 exhibited a prominent month-to-month fluctuations over India, with below normal rainfall in June and August and above normal rainfall in July. The factors determining the month-to-month fluctuations in ISM rainfall during 2016 are investigated with main focus on the Indo-Pacific climatic anomalies. Warm sea surface temperature (SST) anomalies associated with super El Nino 2015 disappeared by early summer 2016 over the central and eastern Pacific. On the other hand, negative Indian Ocean dipole (IOD) like SST anomaly pattern over the equatorial Indian Ocean and anomalous anticyclonic circulation over the western North Pacific (WNP) are reported in summer 2016 concurrently with decaying El Nino/developing La Nina phase. Observations revealed that the low rainfall over central north India in June is due to moisture divergence caused by the westward extension of ridge corresponding to WNP anticyclone and subsidence induced by local Hadley cell partly related to negative IOD. Low level convergence of southeasterly wind from Bay of Bengal associated with weak WNP anticyclone and northwesterly wind corresponding to anticyclonic circulation over the northwest India remarkably contributed to positive rainfall in July over most of the Indian subcontinent. While reduced rainfall over the Indian subcontinent in August 2016 is associated with the anomalous moisture transport from ISM region to WNP region, in contrast to July, due to local cyclogenesis corroborated by number of tropical cyclones in the WNP. In addition to this, subsidence related to strong convection supported by cyclonic circulation over the WNP also resulted in low rainfall over the ISM region. Coupled General Circulation model sensitivity experiments confirmed that strong convective activities associated with cyclonic circulation over the WNP is primarily responsible for the observed negative ISM rainfall anomalies in August 2016. It is noted that the Indo-Western Pacific circulation anomalies in August 2016 are well predicted when the coupled model is initiated with initial conditions from end of July and beginning of August compared to May. This analysis suggests the importance of the WNP circulation in forcing strong sub-seasonal/month to month rainfall variations over India.

Plausible Effect of Weather on Atlantic Meridional Overturning Circulation with a Coupled General Circulation Model

The Atlantic meridional overturning circulation (AMOC) is a vital component of the global ocean circulation and the heat engine of the climate system. Through the use of a coupled general circulation model, this study examines the role of synoptic systems on the AMOC and presents evidence that internally generated high-frequency, synoptic-scale weather variability in the atmosphere could play a significant role in maintaining the overall strength and variability of the AMOC, thereby affecting climate variability and change. Results of a novel coupling technique show that the strength and variability of the AMOC are greatly reduced once the synoptic weather variability is suppressed in the coupled model. The strength and variability of the AMOC are closely linked to deep convection events at high latitudes, which could be strongly affected by the weather variability. Our results imply that synoptic weather systems are important in driving the AMOC and its variability. Thus, interactions between atmospheric weather variability and AMOC may be an important feedback mechanism of the global climate system and need to be taken into consideration in future climate change studies.

Impact of atmospheric model resolution on simulation of ENSO feedback processes: a coupled model study

This study examines El Nino-Southern Oscillation (ENSO)-related air-sea feedback processes in a coupled general circulation model (CGCM) to gauge model errors and pin down their sources in ENSO simulation. Three horizontal resolutions of the atmospheric component (T42, T63 and T106) of the CGCM are used to investigate how the simulated ENSO behaviors are affected by the resolution. We find that air-sea feedback processes in the three experiments mainly differ in terms of both thermodynamic and dynamic feedbacks. We also find that these processes are simulated more reasonably in the highest resolution version than in the other two lower resolution versions. The difference in the thermodynamic feedback arises from the difference in the shortwave-radiation (SW) feedback. Due to the severely (mildly) excessive cold tongue in the lower (higher) resolution version, the SW feedback is severely (mildly) underestimated. The main difference in the dynamic feedback processes lies in the thermocline feedback and the zonal-advection feedback, both of which are caused by the difference in the anomalous thermocline response to anomalous zonal wind stress. The difference in representing the anomalous thermocline response is attributed to the difference in meridional structure of zonal wind stress anomaly in the three simulations, which is linked to meridional resolution.

The Extrapolar SWIFT model (version 1.0): fast stratospheric ozone chemistry for global climate models

Abstract. The Extrapolar SWIFT model is a fast ozone chemistry scheme for interactive calculation of the extrapolar stratospheric ozone layer in coupled general circulation models (GCMs). In contrast to the widely used prescribed ozone, the SWIFT ozone layer interacts with the model dynamics and can respond to atmospheric variability or climatological trends. The Extrapolar SWIFT model employs a repro-modelling approach, in which algebraic functions are used to approximate the numerical output of a full stratospheric chemistry and transport model (ATLAS). The full model solves a coupled chemical differential equation system with 55 initial and boundary conditions (mixing ratio of various chemical species and atmospheric parameters). Hence the rate of change of ozone over 24 h is a function of 55 variables. Using covariances between these variables, we can find linear combinations in order to reduce the parameter space to the following nine basic variables: latitude, pressure altitude, temperature, overhead ozone column and the mixing ratio of ozone and of the ozone-depleting families (Cly, Bry, NOy and HOy). We will show that these nine variables are sufficient to characterize the rate of change of ozone. An automated procedure fits a polynomial function of fourth degree to the rate of change of ozone obtained from several simulations with the ATLAS model. One polynomial function is determined per month, which yields the rate of change of ozone over 24 h. A key aspect for the robustness of the Extrapolar SWIFT model is to include a wide range of stratospheric variability in the numerical output of the ATLAS model, also covering atmospheric states that will occur in a future climate (e.g. temperature and meridional circulation changes or reduction of stratospheric chlorine loading). For validation purposes, the Extrapolar SWIFT model has been integrated into the ATLAS model, replacing the full stratospheric chemistry scheme. Simulations with SWIFT in ATLAS have proven that the systematic error is small and does not accumulate during the course of a simulation. In the context of a 10-year simulation, the ozone layer simulated by SWIFT shows a stable annual cycle, with inter-annual variations comparable to the ATLAS model. The application of Extrapolar SWIFT requires the evaluation of polynomial functions with 30–100 terms. Computers can currently calculate such polynomial functions at thousands of model grid points in seconds. SWIFT provides the desired numerical efficiency and computes the ozone layer 104 times faster than the chemistry scheme in the ATLAS CTM.

A Caveat Note on Tuning in the Development of Coupled Climate Models

AbstractState‐of‐the‐art coupled general circulation models (CGCMs) have substantial errors in their simulations of climate. In particular, these errors can lead to large uncertainties in the simulated climate response (both globally and regionally) to a doubling of CO2. Currently, tuning of the parameterization schemes in CGCMs is a significant part of the developed. It is not clear whether such tuning actually improves models. The tuning process is (in general) neither documented, nor reproducible. Alternative methods such as flux correcting are not used nor is it clear if such methods would perform better. In this study, ensembles of perturbed physics experiments are performed with the Globally Resolved Energy Balance (GREB) model to test the impact of tuning. The work illustrates that tuning has, in average, limited skill given the complexity of the system, the limited computing resources, and the limited observations to optimize parameters. While tuning may improve model performance (such as reproducing observed past climate), it will not get closer to the “true” physics nor will it significantly improve future climate change projections. Tuning will introduce artificial compensating error interactions between submodels that will hamper further model development. In turn, flux corrections do perform well in most, but not all aspects. A main advantage of flux correction is that it is much cheaper, simpler, more transparent, and it does not introduce artificial error interactions between submodels. These GREB model experiments should be considered as a pilot study to motivate further CGCM studies that address the issues of model tuning.

Role of Weddell Sea ice in South Atlantic atmospheric variability

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 74:171-184 (2017) - DOI: https://doi.org/10.3354/cr01495 Role of Weddell Sea ice in South Atlantic atmospheric variability Yushi Morioka1,*, Francois Engelbrecht2,3, Swadhin K. Behera1 1Application Laboratory, JAMSTEC, Yokohama, Japan 2Natural Resources & the Environment, CSIR, Pretoria, South Africa 3School of Geography, Archaeology and Environmental Sciences, University of the Witwatersrand, South Africa *Corresponding author: morioka@jamstec.go.jp ABSTRACT: We investigated the role of Weddell Sea ice in atmospheric variability over the South Atlantic by conducting observational data analysis and coupled general circulation model (CGCM) experiments. Weddell Sea ice exhibits a pronounced interannual variability in early austral summer (November-January). Low sea-ice concentration (SIC) anomalies in the Weddell Sea are strongly associated with anticyclonic atmospheric circulation anomalies in the South Atlantic. Composite analysis reveals that the low SIC anomalies in the Weddell Sea may be attributed to increased solar radiation and northwesterly wind anomalies. The low SIC anomalies, in turn, contribute to warmer skin temperature in the band of 60-70°S and enhance the near-surface atmospheric stability north of this band, implying favorable conditions for sustaining the anticyclonic circulation anomalies in the South Atlantic. This intriguing association between the SIC and atmospheric circulation anomalies is also simulated in CGCM experiments, e.g. when the interannual sea surface temperature variability in the tropics and mid-latitudes is suppressed. These results suggest that Weddell Sea ice, which may undergo interannual variation via air-sea-ice interactions in the high latitudes, influences atmospheric variability over the South Atlantic. KEY WORDS: Air–sea–ice interaction · Interannual variability · South Atlantic · Atmospheric teleconnection · Coupled general circulation model Full text in pdf format Supplementary material PreviousCite this article as: Morioka Y, Engelbrecht F, Behera SK (2017) Role of Weddell Sea ice in South Atlantic atmospheric variability. Clim Res 74:171-184. https://doi.org/10.3354/cr01495 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 74, No. 2. Online publication date: December 27, 2017 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2017 Inter-Research.

Predictability of two types of El Niño and their climate impacts in boreal spring to summer in coupled models

The predictability of the two El Nino types and their different impacts on the East Asian climate from boreal spring to summer have been studied, based on coupled general circulation models (CGCM) simulations from the APEC Climate Center (APCC) multi-model ensemble (MME) hindcast experiments. It was found that both the spatial pattern and temporal persistence of canonical (eastern Pacific type) El Nino sea surface temperature (SST) are much better simulated than those for El Nino Modoki (central Pacific type). In particular, most models tend to have El Nino Modoki events that decay too quickly, in comparison to those observed. The ability of these models in distinguishing between the two types of ENSO has also been assessed. Based on the MME average, the two ENSO types become less and less differentiated in the model environment as the forecast leadtime increases. Regarding the climate impact of ENSO, in spring during canonical El Nino, coupled models can reasonably capture the anomalous low-level anticyclone over the western north Pacific (WNP)/Philippine Sea area, as well as rainfall over coastal East Asia. However, most models have difficulties in predicting the springtime dry signal over Indochina to South China Sea (SCS) when El Nino Modoki occurs. This is related to the location of the simulated anomalous anticyclone in this region, which is displaced eastward over SCS relative to the observed. In boreal summer, coupled models still exhibit some skills in predicting the East Asian rainfall during canonical El Nino, but not for El Nino Modoki. Overall, models’ performance in spring to summer precipitation forecasts is dictated by their ability in capturing the low-level anticyclonic feature over the WNP/SCS area. The latter in turn is likely to be affected by the realism of the time mean monsoon circulation in models.